By now, there's really no oxygen left in the carboy. It's either already been assimilated by the yeast or pushed out by the pressure from the CO2 that's been generated. Shaking won't cause oxidation at this point like adding oxygen from an outside source would, but it'll put yeast back into suspension. I don't think it's necessary for a lager like it might be for a highly flocculant ale yeast like Wyeast 1968.

I do swirl a carboy with a lager in it every couple of days to get some of the CO2 out of the beer and to get more yeast back into the beer, in hopes of drier finishes/better attenuation. Probably not necssary except for big beers, but I can't see that it hurts.

Sometimes when I dry-hop, I grab the carboy by the neck, tilt it, and gently get the beer swirling to make sure the hop flavors and aromas get mixed in well. Did it tonight. Even though there is only CO2 in it, I still try and be careful of not aerating it.